Brochantite

brochantite

antlerite

chalcanthite

bornite

Images

Formula: Cu₄(SO₄)(OH)₆ sulphate with hydroxyl
Crystal System: Monoclinic
Specific gravity: 3.97
Hardness: 3½ to 4
Streak: Green to light green
Colour: Emerald green
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
Environments:

Hydrothermal environments

Brochantite occurs as a secondary mineral in the oxidation zone of high temperature copper ores in acid conditions. When the carbonate concentration is similar to the atmospheric value, at successively higher values of pH (less acid conditions), chalcanthite is replaced by antlerite then brochantite then malachite. Although the environment for brochantite is at a higher pH than chalcanthite and antlerite, it is still on the acid side of neutral. In the UK brochantite is relatively common in low carbonate environments such as the Caldbeck Fells, but much less so in the more carbonate-rich Pennines (JRS 18.12, 13).

Localities

At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the copper ores overlie primary zone mineralisation consisting of quartz-dolomite-sulphide veins hosted in siltstone and schist. Brochantite is widespread throughout the deposit; it formed late in the paragenesis and was found coating azurite, chalcopyrite, covellite, cuprite, goethite, hematite, malachite and pyrite (AJM 22.1.24).

At Jazkazgan, Karaganda Oblast, Kazakhstan, brochantite pseudomorphs after bornite have been found (KL p191).

The type locality is the Mednorudyanskoye Cu Deposit, Nizhnii Tagil, Sverdlovsk Oblast, Russia.

At the Mariquita Mine (Sultana Mine), Usagre, Badajoz, Extremadura, Spain, brochantite is found commonly as microcrystals coating highly altered copper sulphides and associated with oxyplumboroméite, and as small prismatic crystals associated with native mercury and eglestonite  (MinRec 55.4.490).
Brochantite from the Mariquita Mine - Image

Alteration

Primary chalcopyrite readily alters to the secondary minerals bornite, covellite and brochantite.

antlerite and water to brochantite and sulphuric acid
4Cu₃SO₄(OH)₄(s) + 2H₂O(l) → 3Cu₄SO₄(OH)₆(s) + H₂SO₄(aq)
(JRS 18.12, 13)

The Activity-pH diagram below was calculated at 298.2 K for some arsenates and sulphates for constant activity (roughly equivalent to concentration) of H₂AsO₄^- in solution, over a range of values of pH and of SO₄^2- activity (MM 52.689).


The mineral formulae are:
chalcanthite: Cu(SO₄).5H₂O
antlerite: Cu²⁺₃(SO₄)(OH)₄
brochantite: Cu₄(SO₄)(OH)₆
olivenite: Cu₂(AsO₄)(OH)
cornubite: Cu₅(AsO₄)₂(OH)₄
clinoclase: Cu₃(AsO₄)(OH)₃













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